Checksums can be used to protect against accidental modifications to files: if the checksum has not changed, then the file is probably undamaged. The default CRC checksum is not cryptographic.

Cryptographic checksums are those checksums which protect against both accidental modifications and malicious modifications. Use these to verify that there is no trojan inserted into your file. The "md5" algorithm is beginning to show weaknesses against attacks, so "sha1" is preferred.

Examples:

$ cksum /etc/passwd
3052342160 2119 /etc/passwd

Some "cksum" implementations provide other algorithms, such as "md5" and "sha1":